Adsorption four region

The adsorption isotherm of sodium dodecyl sulfate (SDS) on alumina at pH = 6.5 in 0.1 M NaCI (Fig. 4.11a) is characteristic of anionic surfactant adsorption onto a positively charged oxide. As shown by Somasundaran and Fuerstenau (1966) and by Chandar et al. (1987), the isotherm can be divided into four regions. These authors give the following explanation for the adsorption mechanism ... 

The nature of the problem in establishing a mechanistic model of the oxide-electrolyte interface, in which chemical and electrostatic energies are described explicitly, can be appreciated by consideration of the adsorption reaction depicted in Figure 2. The adsorption of a hydrogen ion from the bulk of a monovalent electrolyte is considered. The oxide-solution interface is divided conceptually into four regions the bulk oxide (not shown in the figure), the oxide surface at which the adsorption reaction takes place, the solution part of the double layer containing the counterions, and the bulk of solution. 

The characteristics of surfactant adsorption isotherm on solid surface are generally analysed by the plot of log Ns versus log Ce based on eqn 2.24 or the plot of log T versus log Ce based on eqn 2.25. These plots show four region isotherms as shown in Figure 2.4. 

Figure 2.4 Four region isotherms of surfactant adsorption.

Of these four regions, methane adsorption only occurs in the micropores of an activated carbon. At ambient temperatures, there is negligible methane adsorption in the macropores, and the macropore volume can be considered as void space methane is present in these voids only at the density of the compressed gas. Hence, the volume occupied by macropores and interstitial voids must be minimized for optimum storage capacity, consistent with the need to allow sufficient... 

The four-region adsorption isotherms (Fig. 4.65) can differ in the position of transition points (I/II, II/III, and IlI/IV) and in slopes in the regions II and III (the slopes in regions I and IV are fixed at 1 and 0, respectively), but the dependence of these parameters on the nature of the adsorbate and the adsorbent, and on the experimental conditions has not been fully recognized. Many publications report single adsorption isotherms obtained at certain pH, ionic strength, temperature, equilibration time etc., and systematic studies of the effect of one or more of these parameters on the adsorption isotherm, or involving a series of surfactants or a series of adsorbents are rare. 

A four-region adsorption isotherm for SDS. Slopes in regions I-IIf and transition points between regions are compared with results taken from literature. Solution concentrations at breakpoints reported in different publications are more consistent than corresponding adsorption densities. Uptake of preadsorbed dodecyisulfonate (10 mol dm ) at pH 3 is not influenced by addition of TX-lOO (up to 10 mol dm For 5x 10 mol dm dodecyisulfonate the uptake at pH 3 is enhanced in the presence of TX-100. The potential becomes less positive when TX-100 is added at constant dodecyl sulfonate concentration at pH < 8. 

These forces and hence the stability of the dispersions can be altered/controlled by the adsorption of ions, surfactants, or polymers at the solid-liquid interface. Adsorption of surfactants and polymers at the solid-liquid interface depends on the nature of the surfactant or polymer, the solvent, and the substrate. Ionic surfactants adsorbing on oppositely charged surfaces exhibit a typical four-region isotherm. Such adsorption can alter the dispersion stability mainly by changing the double layer interaction, which depends on the extent of adsorption. Thus, it is seen that alumina suspensions are destabilized by the adsorption of SDS when the zeta potential is reduced to zero. At higher concentrations, bilayered surfactant adsorption can occur with changes in wettability and flocculation of the particles by altering the hydrophobic interactions. 

This is attributed to the failure of the large molecules to enter the pores of the solid. More complex isotherm shapes are encountered as in the case of the adsorption of alkyl surfactants on silica and alumina. For example, the adsorption isotherm of sodium dode-cylsulfonate on alumina consists of four regions depending on the dominant adsorption mechanism. Adsorption of polymeric reagents on minerals typically results in a pseudo-Langmuirian type isotherm as shown in Fig. 4.7 for the adsorption of polyacrylamide on Na-kaolinite (Hollander et al., 1981). 

An isotherm adsorption of charged surfactants on oxides is shown in Fig. 4.10. This isotherm is characterized by four regions, attributed to four different dominant mechanisms being operative in each region. Mechanisms involved in these regions may be viewed as follows ... 

An important case addressed in numerous studies is the adsorption of anionic and cationic surfactants from aqueous solutions on polar surfaces such as metal oxides [5,6]. The shapes of experimental adsorption isotherms, which represent a relationship between the adsorption, T, and the equilibrium surfactant concentration, c, have been thoroughly investigated and some common features noted. A typical adsorption isotherm plotted on a log-log scale can be subdivided into four regions ( Fig. III-8, a), the interpretation of which and... 

Fig. III-8. A model S-shaped four-region isotherm for the adsorption of ionic surfactant on an oppositely charged surface (a) The structure of the adsorbed surfactant layers corresponding to different regions of the adsorption isotherm (b)...

Fig. 4.23 Peak intensities as a function of CO dosage. The shapes of the curves point to four regions with different behavior in the adsorption process (the details are discussed in the text). Reprinted with permission from [3]. Copyright (1999) by the American Physical Society...

The surfactant adsorption isotherm is usefully divided into four regions. Region I is called the Henry s Law... 

Four regions can be distinguished according to the proportion of precipitant in the developer in the thin-layer chromatography of polymers (Figure 9-20). Adsorption predominates at low precipitant contents. The... 

The mechanisms driving surfactant adsorption are generally discussed in terms of the four-region isotherms. At low surfactant concentrations, designated as region I (see Figure 2), the adsorption behavior can usually... 

Typical four-region adsorption isotherm for a monoisomeric... 

For nonlinear systems, however, the evaluation of the flow rates is not straightforward. Morbidelli and co-workers developed a complete design of the binary separation by SMB chromatography in the frame of Equilibrium Theory for various adsorption equilibrium isotherms the constant selectivity stoichiometric model [21, 22], the constant selectivity Langmuir adsorption isotherm [23], the variable selectivity modified Langmuir isotherm [24], and the bi-Langmuir isotherm [25]. The region for complete separation was defined in terms of the flow rate ratios in the four sections of the equivalent TMB unit ... 

---